TheAlgorithms/Java · error · IllegalArgumentException

Eviction strategy must not be null

Error message

Eviction strategy must not be null

What it means

Builder.evictionStrategy() rejects null because it needs a concrete EvictionStrategy to decide when to clean up expired entries. If you never call this method, the builder defaults to PeriodicEvictionStrategy(100). Passing null is always a programming error.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/caches/RRCache.java:499

        /**
         * Builds and returns a new {@link RRCache} instance with the configured parameters.
         *
         * @return a fully configured {@code RRCache} instance
         */
        public RRCache<K, V> build() {
            return new RRCache<>(this);
        }

        /**
         * Sets the eviction strategy used to determine when to clean up expired entries.
         *
         * @param strategy an {@link EvictionStrategy} implementation; must not be {@code null}
         * @return this builder instance
         * @throws IllegalArgumentException if {@code strategy} is {@code null}
         */
        public Builder<K, V> evictionStrategy(EvictionStrategy<K, V> strategy) {
            if (strategy == null) {
                throw new IllegalArgumentException("Eviction strategy must not be null");
            }
            this.evictionStrategy = strategy;
            return this;
        }
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Do not call evictionStrategy() if the default PeriodicEvictionStrategy(100) is acceptable
2. Use NoEvictionStrategy for per-access cleanup, or provide a custom implementation
3. Guard the call so it only fires when the strategy is non-null

Example fix

// before
EvictionStrategy<String,String> strat = strategyFactory.get(); // may be null
new RRCache.Builder<String,String>(100).evictionStrategy(strat).build();

// after
RRCache.Builder<String,String> b = new RRCache.Builder<>(100);
EvictionStrategy<String,String> strat = strategyFactory.get();
if (strat != null) b.evictionStrategy(strat);
b.build();

// Omit evictionStrategy() to use the default PeriodicEvictionStrategy(100).
// Only call with a concrete implementation:
if (strategy != null) {
    builder.evictionStrategy(strategy);
} else {
    builder.evictionStrategy(new RRCache.NoEvictionStrategy<>());
}

if (strategy instanceof RRCache.EvictionStrategy) {
    builder.evictionStrategy((RRCache.EvictionStrategy<K,V>) strategy);
}

try {
    builder.evictionStrategy(strategy);
} catch (IllegalArgumentException e) {
    builder.evictionStrategy(new RRCache.PeriodicEvictionStrategy<>(100));
}

When it happens

Trigger: Calling builder.evictionStrategy(null), or passing a strategy variable that was never assigned.

Common situations: Intending to use the default strategy but calling the method with null instead of omitting it. Passing a strategy from a factory that can return null on misconfiguration.

Related errors

• Random must not be null
• Listener must not be null
• Key must not be null
• Key and value must not be null
• Interval must be > 0

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/7c31096ee61f9182. Report an issue: GitHub.